Chirped pulse amplification is a general method for obtaining maximum ultrashort pulse energies from an optical amplifier (see, Compression of Amplified Chirped Optical Pulses, D. Strickland and G. Mouro, Opt. Commun. 56, 219 (1985)). Maximum pulse energy in a particular amplifying medium is determined by the value of the saturation fluence. However, for a propagating ultrashort optical pulse, unacceptably high peak intensity can be reached at pulse fluencies well below the saturation fluence of the amplification material. Therefore, for the majority of amplifying media it is necessary to stretch ultrashort pulses prior to amplification. The initial ultrashort duration must be restored after the amplification by recompressing the stretched pulse. Use of chirped pulse amplification allows a reduction in the peak intensities of pulses in the amplifier and avoidance of nonlinear distortions in ultrashort pulses.
A rare-earth doped fiber amplifier is one example of an amplifying medium which provides exceptional technological advantages. A fiber-based laser or amplifier can be directly pumped by a laser diode, and a compact and robust device can be designed therefrom. The value of the saturation fluence in single-mode Er-doped fibers permits optical pulses with energies in the 10 to 100 .mu.J range. However, direct amplification of subpicosecond optical pulses in, e.g. Er-doped fiber, is possible only for energies of up to approximately the nanojoule level. In order to take advantage of the full capability of the fiber, it is necessary to stretch optical pulses prior to amplification up to the duration of approximately 0.1 ns to 1 ns or longer.
In the conventional implementation of chirped pulse amplification, diffraction-grating-based ultrashort-pulse stretchers and compressors are used, as described by P. Maine et al. in Generation of Ultrahigh Peak Power Pulses by Chirped Pulse Amplification, IEEE J. Quantum Elect. Vol. 24, No. 2, Feb. 1988. These diffraction-grating arrangements are particularly useful for recompressing high energy pulses, because diffraction gratings are not susceptible to optical damage and nonlinear effects up to very high peak intensities. Recompression of ultrashort pulses with energies above the 1 J level have been achieved with such pulse compressors. The disadvantage of diffraction-grating stretchers and compressors is the large size of these arrangements. For stretching pulses to nanosecond durations and longer, diffraction-grating based systems can reach several meters in length. Even the most compact arrangements typically have a length of tens of centimeters. Additionally, exact compensation of the pulse phase requires very precise alignment of the arrangement, which is not a trivial task. The alignment requirements make such devices unsuitable for mass production.
Chirped Bragg-grating pulse stretchers and compressors have been proposed recently as a compact replacement for bulk diffraction-grating arrangements (See, U.S. Pat. No. 5,499,134, referred to hereinafter as "the '134 patent"). Chirped gratings can be directly written into the core of an optical fiber. Very compact arrangements with chirped fiber Bragg-gratings can deliver longer stretched pulses compared to diffraction-grating devices. The length L of a chirped fiber-grating is directly related to the duration AT of a stretched pulse: .DELTA..tau.=2L/v.sub.g (provided that spectral bandwidths of the pulse and the grating are perfectly overlapped). Here v.sub.g is the group velocity of light in the grating fiber. For example, nanosecond stretched pulses can be obtained with a 10 cm long fiber-grating. Such chirped fiber Bragg-grating devices are ideally compatible with fiber amplifier technology and provide compact implementations of all-fiber chirped pulse amplification techniques (See, A. Galvanauskas et al., All-fiber Femtosecond Pulse Amplification Circuit Using Chirped Bragg Grating, Appl. Phys. Lett., Vol. 66 (9), Feb. 27, 1995). One essential technological advantage of fiber-grating devices is that, similar to semiconductor circuit technology, such devices can be mass-produced at relatively low cost by using phase-mask technology.
Chirped fiber-grating compressors can produce pulsed outputs with substantially higher energies than direct amplification in an optical fiber. Fiber gratings have 4 to 6 orders of magnitude higher dispersion compared to a standard fiber. Consequently, the propagation length for a recompressed pulse in a grating fiber is substantially shorter than in an amplifier fiber and nonlinear effects occur at substantially higher peak powers.
However, the obtainable energies from a fiber-grating compressor are limited compared to a diffraction-grating compressor. The essential disadvantage is that an all-fiber chirped pulse amplification (CPA) system based on a fiber-grating compressor can provide pulses with maximum energies only of about 100 nJ, which is more than 100 times lower than the saturation-fluence limit for a fiber amplifier. Therefore, using a fiber-grating compressor effectively prevents full utilization of the fiber amplifier.
There are a number of applications where use of high pulse energies is essential. One example is a fiber amplifier CPA-based optical parametric generation system for producing a broadly tunable wavelength output. Optical parametric generation can be achieved only with pump energies exceeding a certain threshold, which typically is higher than can be delivered with a fiber-grating compressor. This energy requirement makes it necessary to resort to diffraction-grating compressors, which are disadvantageous for the reasons described above.
One solution would be to use bulk chirped Bragg-grating devices which could accept large beams and, subsequently, to eliminate nonlinear-effect limitations. However, such devices have not yet been achieved due to the substantial technological problems of writing a Bragg-grating with the required characteristics into a large volume of photosensitive material.
A partial solution was suggested in U.S. Pat. No. 5,696,782.
Disadvantages of using a diffraction-grating compressor can be partially alleviated if a hybrid scheme is used. In the '782 patent, the fiber CPA system includes a fiber-grating stretcher and compressor and an additional diffraction-grating compressor immediately following the fiber-grating compressor. The function of the fiber-grating compressor is to compress the amplified pulses only partially in order to reduce the size of the diffraction-grating compressor and to circumvent the nonlinear-effect limitations of the fiber-grating compressor. One disadvantage of this scheme is that technological problems related to the size and alignment of the diffraction-grating device are merely reduced, but not eliminated. Another disadvantage is that pulse energy losses are typically high after passing two compressors. These losses increase if a frequency conversion, such as a second-harmonic generation, is required after the pulse compression (e.g. for pumping and optical parametric generation).
In a chirped quasi-phase-matched (QPM) crystal, the ability to perform chirp compensation and frequency conversion is based on two key features of the crystal. First, as is typical for any nonlinear material, group velocities of the fundamental-wavelength pulse and of the second-harmonic pulse are different along the same propagation path, resulting in temporal walk-off between these two pulses. Second, in a chirped QPM crystal, second-harmonic generation for different input wavelengths is localized at different spatial positions along the pulse propagation path. As a result, a bandwidth-limited pulse at the fundamental wavelength, launched into such a crystal, will produce second-harmonic pulses with a frequency chirp. The duration of this second-harmonic (SH) pulse .DELTA.T is determined by the magnitude of the group-velocity walk-off: .DELTA.T=L/v.sub.SH -L/v.sub.Fund. Here L is the length of the crystal and v.sub.SH, v.sub.Fund are the group-velocities at second-harmonic and fundamental harmonics, respectively. The frequency-bandwidth .DELTA.v of the SH pulse is given by the magnitude of the QPM period variation (QPM chirp bandwidth). Equivalent frequency chirp of an optical pulse can be produced by group-velocity dispersion equal to .DELTA.T/.DELTA.v. Note that, for the two opposite directions of propagation, the frequency chirp of the second-harmonic pulses produced therefrom have opposite signs.
However, QPM crystals cannot be used as direct replacements for conventional pulse stretchers and compressors. One essential difference is that they do not provide any actual group-velocity dispersion (ignoring the small intrinsic dispersion of a nonlinear material). Therefore, fundamental pulses cannot be stretched, as is necessary for implementing a CPA scheme. Also, due to the limited values of the group-velocity walkoff and due to the technological limitations on the achievable QPM crystal lengths, the amount of stretched-pulse compensation on the SH pulse is substantially smaller than that achievable with fiber-grating or diffraction-grating compressors.
In U.S. patent application Ser. No. 08/824,032, Arbore et al. disclose that a quasi-phase-matched (QPM) second harmonic generator (SHG) with QPM period chirped along the crystal length, in effect, provides group velocity dispersion (GVD) at the second-harmonic wavelength. This property allows construction of unique devices for simultaneous second-harmonic generation and temporal stretching or compression of the second-harmonic output with respect to the fundamental input pulses.
In U.S. Pat. No. 5,696,782, A. Galvanauskas, M. Arbore, M. Fejer and D. Harter disclose the use of a chirped QPM element in a CPA system. The present application relates to the use of chirped QPM elements in fiber-based systems.
All of the above-mentioned articles, patents and patent applications are hereby incorporated herein by reference.